[RedDevil]

Originally Posted by euromodder

It's kind of what I've been thinking about - only reversed.

You don't want a sphere at the rear of a vehicle - it'll add drag, and cause instability when fitted on a trailer.

Now, add the sphere on the trailer's front.
That'd be good for FE, even without gap filling panels.
In a simpler form and providing more internal space, it could just as well be a half cylinder with a slanted top rather than a sphere...

I was not thinking of a real sphere. That is just the dividing 'line' (convex plane) between tow and trailer.
Just some panels outlining the sides and roof of the imaginary sphere would do.
The trailers side panels would be very close to that; that is the whole reason for this setup. So there should be no aerodynamic instability whatsoever.
Tow vehicle and trailer would form one body with just a very small gap in between.

The panels need to be removable or you would not be able to reach the hitch point.

The location of the sphere is solely dependant on the location of the hitch point.
With the hitch at the back the tow vehicle needs to stay within the sphere.
An overhead hitch point like a semi would allow for the front of the trailer bearing the sphere or have top and side panels reaching to the outline of the sphere, while the cabin would have the hollow end (or matching panels).

A close-fitting cylinder and countercylinder would work in corners on the flat, but hit each other when the road is not level, like concave or askew.
When the divide is a sphere around the hitch point they would never hit.

It is easy to determine where the gap should be - tie a rope to the hitch point, find what lenght you need to reach the outermost points of whatever needs to be within the sphere and cut the panels wherever you can stretch the rope for that length. Then hitch the trailer in line and fit the panels on precisely so that they just don't hit.